The reason why ST connectors (it's not a format or protocol) never made it in audioland is that ST connectors are not very consumer friendly, TOSLINK is.
You can even make your own cables and mount connectors yourself for TOSLINK. ST' consumers' can not (well I can... but not at home).
Also you cannot (and could not) buy ST patch cords (of various lengths and diameters) in your local audio shop, they would have to be ordered from specialised stores.

It's like the Elcassette, DCC, videodisc etc, good ideas, but no wide support by various brands tends to kill initiatives.